In output signals (such as output voltages) output from electronic devices, such as a switching power supply, noise having the same frequency as the operation frequency of the electronic device or harmonic frequencies thereof may be mixed. Because the noise may have an adverse effect on an external electronic device, it is necessary to reduce the noise. For example, a switching power supply outputs an output voltage of a predetermined voltage value by a switching operation of a power transistor. As the current paths are switched by the turning on and off of the power transistor, noise having the switching frequency and harmonic frequencies thereof may be generated. The noise may become superposed on the output voltage and be input into an external electronic device. In order to decrease the noise, a noise reduction device provided with an inductance element, such as a choke coil, may be inserted in an output path for the output voltage (see, for example, JP-A-2015-57806).
In the automotive field, for example, the electronic devices such as the switching power supply described above may be accommodated in a metal housing made of aluminum and the like, from the viewpoint of ensuring reliability in terms of not being affected by the surrounding environment, such as vibrations. For the same purpose of ensuring reliability, it has also been proposed to accommodate the noise reduction device in a metal housing. However, the metal housing does not necessarily ensure that the problems caused by the circuit elements or wiring, such as electromagnetic coupling including capacitive coupling and inductive coupling, and sneak noise from the ground potential, are sufficiently prevented in the housing. Accordingly, a noise caused by the switching operation of a power transistor, for example, may reach the output terminal without passing through the output path of the noise reduction device.
In order to address the problem, a part of the output side of a noise reduction device disclosed in JP-A-2015-57806 is disposed outside the metal housing. The noise reduction device is provided with a choke coil module. The choke coil module is obtained by sealing a member, including a conductive bar inserted into a tubular ferrite core, with thermosetting resin. A flange portion of the choke coil module is disposed on the outside of the metal housing. The flange portion includes a vertical plane with respect to the direction in which the conductive bar extends. The plane is opposed to an outer peripheral surface of the metal housing, and affixed to the metal housing by means of bolts. In the flange portion, a substrate with chip capacitors mounted thereon is sealed with resin. The chip capacitors are connected to a choke coil in the LC filter circuit.